The present invention relates generally to the field of proteomics and more specifically to quantitative analysis of glycoproteins.
Complete genomic sequences and large partial (EST) sequence databases potentially identify every gene in a species. However, the sequences alone do not explain the mechanism of biological and clinical processes because they do not explain how the genes and their products cooperate to carry out a specific process or function. Furthermore, the gene sequence does not predict the amount or the activity of the protein products nor does it answer the questions of whether, how, and at what position(s) a protein may be modified.
Quantitative protein profiling has been recognized as an important approach for profiling the physiological state or pathological state of cells or organisms. Specific expectations of quantitative protein profiles include the possibility to detect diagnostic and prognostic disease markers, to discover proteins as therapeutic targets or to learn about basic biological mechanisms.
Not only do the amounts and type of proteins expressed vary in different pathological states, post-translational modifications of proteins also vary depending on the physiological or pathological state of cells or organisms. Thus, it is important to be able to profile the amount and types of expressed proteins as well as protein modifications.
Glycosylation has long been recognized as the most common post-translational modification affecting the functions of proteins, such as protein stability, enzymatic activity and protein-protein interactions. Differential glycosylation is a major source of protein microheterogeneity. Glycoproteins play key roles in cell communications, signaling and cell adhesion. Changes in carbohydrates in cell surface and body fluid are demonstrated in cancer and other disease states and highlights their importance. However, studies on protein glycosylation have been complicated by the diverse structure of protein glycans and the lack of effective tools to identify the glycosylation site(s) on proteins and of glycan structures. Oligosaccharides can be linked to serine or threonine residues (O-glycosylation) or to asparagine residues (N-glycosylation), and glycoproteins can have different oligosaccharides attached to any given possible site(s).
Among the many post-translation modifications of proteins, glycosylation is a modification that is common to proteins that are exposed to an extracellular environment. For example, proteins expressed on the surface of a cell are exposed to the external environment such as blood or surrounding tissue. Similarly, proteins that are secreted from a cell, for example, into the bloodstream, are commonly glycosylated.
Among the diverse types of proteins expressed by cells, proteins that are integral to or associated with lipid membranes perform a wide range of essential cellular functions. Pores, channels, pumps and transporters facilitate the exchange of membrane impermeable molecules between cellular compartments and between the cell and its extracellular environment. Transmembrane receptors sense changes in the cellular environment and, typically via associated proteins, initiate specific intracellular responses. Cell adhesion proteins mediate cell-specific interactions with other cells and the extracellular matrix. Lipid membranes also provide a hydrophobic environment for biochemical reactions that is dramatically different from that of the cytoplasm and other hydrophilic cellular compartments.
Membrane proteins, in particular those spanning the plasma membrane, are also of considerable diagnostic and therapeutic importance, which is further reinforced due to their easy accessibility. Antisera to proteins that are selectively expressed on the surface of a specific cell type have been used extensively for the classification of cells and for their preparative isolation by fluorescent activated cell sorting or related methods. Membrane proteins, as exemplified by Her2/neu, the abundance of which is modulated in the course of certain diseases such as breast cancer, are commonly used as diagnostic indicators and, less frequently, as therapeutic targets. A humanized monoclonal antibody (Herceptin, Genentech, Palo Alto, Calif.) that specifically recognizes Her2/neu receptors is the basis for a successful therapy of breast cancer, and antibodies to other cell surface proteins are also undergoing clinical trials as anticancer agents. Moreover, the majority of current effective therapeutic agents for diseases such as hypertension and heart disease are receptor antagonists that target and selectively modify the activity of specific membrane proteins. It is therefore apparent that a general technique capable of systematically identifying membrane proteins and of accurately detecting quantitative changes in the membrane protein profiles of different cell populations or tissues would be of considerable importance for biology and for applied biomedical research.
In addition to membrane bound proteins, proteins secreted by cells or shed from the cell surface, including hormones, lymphokines, interferons, transferrin, antibodies, proteases, protease inhibitors, and other factors, perform critical functions with respect to the physiological activity of an organism. Examples of physiologically important secreted proteins include the interferons, lymphokines, protein and peptide hormones. Aberrant availability of such proteins can have grave clinical consequences. It is therefore apparent that the ability to precisely quantitatively profile secreted proteins would be of great importance for the discovery of the mechanisms regulating a wide variety of physiological processes in health and disease and for diagnostic or prognostic purposes. Such secreted proteins are present in body fluids such as blood serum and plasma, cerebrospinal fluid, urine, lung lavage, breast milk, pancreatic juice, and saliva. For example, the presence of increased levels of prostate-specific antigen has been used as a diagnostic marker for prostate cancer. Furthermore, the use of agonists or antagonists or the replacement of soluble secreted proteins is an important mode of therapy for a wide range of diseases.
Quantitative proteomics requires the analysis of complex protein samples. In the case of clinical diagnosis, the ability to obtain appropriate specimens for clinical analysis is important for ease and accuracy of diagnosis. As discussed above, a number of biologically important molecules are secreted and are therefore present in body fluids such as blood and serum, cerebrospinal fluid, saliva, and the like. In addition to the presence of important biological molecules, body fluids also provide an attractive specimen source because body fluids are generally readily accessible and available in reasonable quantities for clinical analysis. It is therefore apparent that a general method for the quantitative analysis of the proteins contained in body fluids in health and disease would be of great diagnostic and clinical importance.
A key problem with the proteomic analysis of serum and many other body fluids is the peculiar protein composition of these specimens. The protein composition is dominated by a few proteins that are extraordinarily abundant, with albumin alone representing 50% of the total plasma proteins. Due to the abundance of these major proteins as well as the presence of multiple modified forms of these abundant proteins, the large number of protein species of lower abundance are obscured or inaccessible by traditional proteomics analysis methods such as two-dimensional electrophoresis (2DE).
The classes of proteins described above, membrane proteins, secreted proteins, and proteins in body fluids have in common that they have a high propensity for being glycosylated, that is, modified post translationally with a carbohydrate structure of varying complexity at one or several amino acid residues. Thus, the analysis of glycoproteins allows characterization of important biological molecules.
Thus, there exists a need for methods of high throughput and quantitative analysis of glycoproteins and glycoprotein profiling. The present invention satisfies this need and provides related advantages as well.